MEPS 124:289-299 (1995) - doi:10.3354/meps124289

Simple procedure for simultaneous recovery of dissolved inorganic and organic nitrogen in 15N-tracer experiments and improving the isotopic mass balance

Slawyk G, Raimbault P

We developed a simple and reliable method which allows simultaneous isotope-ratio analysis of inorganic (DIN) and organic (DON) forms of nitrogen extracted from seawater. All forms of nitrogen under analysis are converted to ammonium, by diffusion
with magnesium oxide, prior to collection on glass-fiber filters appropriate for mass spectrometric assay of 15N. Oxidized DIN forms (nitrate, nitrite) are reduced to ammonium in the presence of Devarda alloy. Conversion of DON to ammonium is
performed by wet oxidation using potassium persulfate and subsequent reduction of the nitrate formed. Recovery tests, both for total nitrogen and 15N content, showed that this procedure is suitable for application in DI15N-isotope
dilution experiments and DON-release studies. Recovery of total nitrogen from DIN and DON was nearly complete (94 to 97%). The variability in the experimental determination of 15N abundance was &LT2% and &LT4% for DIN and DON, respectively. We
used the method to balance the 15N budget in nitrate and ammonium uptake experiments conducted in an oligotrophic area (tropical North Atlantic) by including, in addition to the substrate (DIN) and biomass (PON) pool, the DON pool. However, the
use of glass-fiber filters (GF/F) for the collection of particulate matter produced a significant artifact, i.e. a large amount of small particles (&LT0.7 um, PON&LTGF/F; prochlorophytes and/or bacteria) passed through these filters and were
recovered together with the DON in a combined pool. While inclusion of this combined pool led virtually to a complete accounting for the 15N label (99%) in all samples for nitrate uptake and in those for ammonium uptake incubated for &LT8 h, no
mass balance was achieved during ammonium uptake lasting 10 to 24 h. We suggest that the 15N that was still missing (13%) resulted mainly from bottle containment effects such as ammonium-ion adsorption and/or PON adherence onto incubation
bottle walls. Transfer of 15N label to the combined pool (nitrate experiment) and to the DON and PON&LTGF/F pools (ammonium experiment) represented up to 41, 38 and 20% of the total 15N taken up as DIN, respectively, and
depended strongly upon the length of incubation. Failure to take these pathways of the missing 15N into account during traditional 15N uptake experiments involves risk of substantially underestimating new and regenerated production,
at least in oligotrophic areas. The latter fact has considerable significance in the design of future 15N tracer methodologies.